JPS6237110B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a spring steel with excellent resistance to fatigue. Conventionally, SUP6 and SUP9 have been the main spring steels used in suspension systems for automobiles and the like. In recent years, there has been a demand for lighter automobiles, and there has also been a strong demand for lighter suspension systems themselves. In response to this problem, various measures have been attempted for suspension systems in general, and among these measures, a measure of increasing the spring design stress is considered to be effective. As described above, with the high stress design, when a spring is manufactured using the above-mentioned conventional spring steel as a material, a problem arises in that the sag increases. Particularly when used in passenger cars, increased sag leads to a reduction in bumper height, posing a major safety problem. As a result of various studies, it was discovered that increasing the Si content in spring steel improves the fatigue resistance. Among all spring steels, SUP7, which has the highest Si, has come to be widely used as a steel for passenger car suspension springs. However, there are strict requirements for reducing the weight of suspension springs, and there has been a strong desire to develop a spring steel that is even more resistant to fatigue than SUP7. Against this background, the present invention was developed as a result of repeated research by the present inventors, by adding an appropriate amount of V, V, and Nb to high-Si spring steel, and further adding one of Cu, Co, and Be. In addition to addition, depending on the purpose of use
Contains Cu, Al, or Co, Ti, or
By adding one of B, Cr, or Ta, it has better fatigue resistance and hardenability than SUP7, and also has the same performance as SUP7 in terms of fatigue resistance and toughness, which are necessary for spring steel. This is a steel for springs that has been developed. V, Nb, and Ta each form carbides in steel, and these alloy carbides dissolve into austenite during heating during quenching. When this is rapidly cooled and quenched, martensite containing these elements in a supersaturated solid solution is obtained. When this is tempered, fine alloy carbides begin to re-precipitate in the process, which prevents the movement of dislocations in the steel, causing secondary hardening, making the steel harder than spring steel without the addition of V and Nb. It also works to improve the resistance to fatigue. In addition, alloy carbides that are not dissolved in austenite during heating during quenching can refine austenite crystal grains and prevent them from becoming coarser. Moreover, such fine crystal grains improve the resistance to settling by reducing the amount of movement of dislocations. In addition, Cu, Co, and Be form a substitutional solid solution in steel, similar to Si, and impart solid solution strengthening to the steel, thereby improving the steel's resistance to settling. On the other hand, Al and Ti often contain N in steel.
It combines with nitrides to form nitrides, which has the effect of refining austenite crystal grains and preventing their coarsening, and contributes to improving the settling resistance of steel by reducing the amount of dislocation movement. In addition, B and Cr are elements that improve the hardenability of steel, and the added elements can improve the fatigue resistance of thick springs that require particularly high hardenability. It is something. The fact that secondary hardening occurs means that when aiming for the same tempering hardness range, the tempering temperature range can be wider than that of conventional steel, and the targeted hardness can be stably achieved. It turns out. To further clarify this, we used A15 steel containing 0.22% V, 0.18% Ta, and 1.33% Cu, which will be described later.
0.21% V, 0.12% Nb, 0.20% Ta, and
A16 steel containing 1.30% Cu and B1 steel of SUP7 were tempered at 300 to 650°C, and the hardness was measured. The results are shown in Figure 1. As is clear from Fig. 1, A15 containing precipitation-strengthening elements V, Nb, Ta, and Cu as appropriate;
In the A16 steel of the present invention, it is recognized that the tempering temperature range corresponding to hardness is wider than that of conventional steel, and at the same time, an increase in hardness indicating the occurrence of secondary hardening occurs at a tempering temperature of 550°C. You can see it. Next, regarding the grain refinement effect, 0.21%
A7 containing V, 1.31% Cu, 0.048% Al
Steel, 0.19% V, 0.12% Nb, 1.25% Cu, and
A8 steel containing 0.057% Al, 0.22% V, 0.59
Each austenite of A9 steel containing % Co, 0.07% Ti, A10 steel containing 0.20% V, 0.14% Nb, 0.68% Co, 0.05% Ti and B1 steel which is the conventional steel SUP7 The results of measuring the austenite grain size at the oxidation temperature using the oxidation method are shown in the second
Shown in the figure. Looking at Figure 2, it can be seen that due to the addition of Al or Ti, the austenite grain size is about 3 finer in grain size number than that of conventional steel. In addition, Figure 3 shows 0.25% V, 1.30% Cu,
A11 steel containing 0.0030% B, 0.23% V,
A12 steel containing 0.10% Nb, 1.27% Cu, 0.0021% B, 0.27% V, 1.34% Cu, 0.62% Cr
A13 steel containing 0.21% V, 0.15% Nb,
The hardenability is compared between A14 steel containing 1.29% Cu and 0.65% Cr and B1 steel, which is a conventional steel SUP7. This figure shows that the hardenability is significantly improved by B or Cr. The chemical composition of the steel of the present invention is C0.50-0.80%, Si1.50
~2.50%, Mn0.50~1.50%;
V0.05~0.50% or V0.05~0.50%, Nb0.05
Contains ~0.50% and further Cu0.20~3.00%,
Contains one of Co0.59~1.00%, Be1.10~2.00%, and Cu0.20~3.00% depending on the purpose of use.
Al0.03~0.10% or Co0.59~1.00%,
Contains Ti0.02~0.10%, and B0.0005~
Contains one of the following: 0.0100%, Cr 0.20 to 1.00%, or Ta 0.05 to 0.50%, and the rest is substantially Fe.
It is more than that. The reasons for limiting the composition of the steel of the present invention will be explained below. The reason why the C content is set at 0.50 to 0.80% is that if it is less than 0.50%, sufficient strength cannot be obtained as a steel for high stress springs through quenching and tempering. This is because the decrease in toughness becomes significant. The reason why the amount of Si is set at 1.50 to 2.50% is because if it is less than 1.50%, the effect of increasing the strength of the base material and improving the resistance to settling cannot be obtained sufficiently by solid solution of Si in the ferrite. This is because even if the content exceeds 2.50%, the effect of improving the resistance to settling is saturated, and there is a risk that free carbon may be generated by heat treatment. The reason for setting the Mn content to 0.50 to 1.50% is that if it is less than 0.50%, the strength as a spring steel is insufficient, and the hardenability is also insufficient.If the Mn content exceeds 1.50%, the toughness This is to inhibit the Both V and Nb are elements that improve the sag resistance in the steel of the present invention. The reason why we set the content of V and Nb, which have these functions, at 0.05 to 0.50% each is because the above effects cannot be obtained sufficiently below 0.05%.
Even if the content exceeds %, the effect will be saturated, and the amount of alloy carbides that are not dissolved in austenite will increase, forming large lumps, which may reduce the fatigue strength of steel due to the action of nonmetallic inclusions. This is because there is. These V and Nb can be added individually or by adding the two in combination.
Compared to when Nb is added alone, dissolution into austenite starts at a lower temperature, and the precipitation of fine alloy carbides during the tempering process is
By further promoting secondary curing, the sagging resistance is further improved. Further, Cu, Co, and Be are elements that form a solid solution in the steel in a substitutional manner, strengthen the steel, and improve the resistance to settling. The reason why the Cu content is set to 0.20 to 3.00% is because if it is less than 0.20%, solid solution strengthening is insufficient, and if it is added in excess of 3.00%, there is a possibility that hot rollability may be inhibited. In addition, the reason why the Co content was set to 0.59 to 1.0% is that
This is because if the content is less than 0.59%, the effect is insufficient, and if it exceeds 1.00%, the toughness may deteriorate. Similarly, the Be content was set to 1.10 to 2.00% because
This is because although Be is an element with a large solid solution strength ability, the above effect cannot be obtained if it is contained below 1.10%, and even if it is contained in excess of 2.00%, the effect is saturated as in the case of Si. In addition, the content of Al and Ti, which refine grains and improve fatigue resistance, is 0.03 to 0.10 for Al.
% and Ti are set at 0.02 to 0.10% because if the content is less than that, the distribution state of these nitrides becomes sparse and they do not contribute to grain refinement, and even if the content exceeds 0.10%, the above This is because there is a risk that the effect of saturated steel may reach saturation, impede hot rolling properties, and deteriorate the toughness of the steel as nonmetallic inclusions. In addition, B and Cr improve hardenability, and when hardened, thick springs have a martensitic structure down to the center, resulting in precipitation strengthening, solid solution strengthening,
It is an element that has the effect of fully demonstrating the effect of refining crystal grains. The content of B was set to 0.0005 to 0.0100% because
This is because if the content is less than 0.0005%, no improvement in hardenability can be expected, and if the content exceeds 0.0100%, the effect will be saturated. Cr content over 0.20~1.00
% because if it is less than 0.20%, the effect of improving hardenability is not sufficient, and if it exceeds 1.00%, the effect of improving hardenability is almost saturated, and the This is because steel containing a large amount of Si may have a non-uniform tempered structure. Further, like V and Nb, Ta is an element that forms carbides in steel and reprecipitates during tempering, thereby imparting precipitation strengthening to steel. In addition, the Ta content was set at 0.05 to 0.50% because at 0.05% or less,
This is because the amount of precipitation is insufficient, and even if the content exceeds 0.50%, the effect will be saturated, and undissolved carbides may act as nonmetallic inclusions and deteriorate the toughness of the steel. In this case as well, in addition to adding Ta alone, it exerts a synergistic effect with the already added V and Nb to start dissolving alloy carbides at a lower austenitizing temperature and further promote secondary hardening. It is something. Next, the characteristics of the steel of the present invention will be clarified through examples in comparison with conventional steel. Table 1 shows the chemical composition of these test steels.

[Table] In Table 1, A1 to A18 steels are the steels of the present invention, and B1
The steel is conventional steel and is SUP7. All of these were cast and then hot rolled at a rolling ratio of 50 or higher to form test materials. Then, a coil spring having the specifications shown in Table 2 was formed using the above-mentioned test steel as a raw material, and the final hardness was
After quenching and tempering the wire to give an HRC of 45 to 55, the wire was set to a shear stress τ of 115 Kg/mm ² to prepare a fatigue test piece. Then, at a constant temperature of 20℃, this test piece was subjected to a load such that the shear stress of the wire was τ = 105Kg/ ^mm2 , and the amount of fatigue of the coil spring after 96 hours (hereinafter referred to as long-term load) was measured.

【table】

[Table] However, for A11 to A14 steels in consideration of hardenability, test pieces were formed into torsion bars with a diameter of 30 mm as shown in Table 3, and after setting at τ = 110 Kgf/mm ² , τ = A stress of 100 Kgf/mm ² was applied and the amount of settling was determined after being left for 96 hours.

[Table] Figures 4 to 8 show the amount of set in relation to the hardness of the test piece. As is clear from Figures 4 to 8, it is the steel of the present invention.
It was recognized that all of the A1 to A18 steels had superior fatigue resistance compared to the conventional steel B1 steel. The amount of setback is the load required to compress the coil spring to a certain height before applying the long-term load.
Measure P ₁ and the load P ₂ required to compress to the same height after applying the long-term load, and calculate the difference △P
(=P ₁ −P ₂ ) using the following formula, and has a unit of shear strain, and was evaluated using a value called residual shear strain. γR=1/G・K8D/d ³ △PG G: Transversality rate (Kgf/mm ² ) D: Coil center diameter (mm) d: Wire diameter (mm) K: Whirl correction coefficient (coil spring shape (a constant determined by
(rad) was changed to the amount of residual shear strain according to Yr=△θ・d/2l. d: Wire diameter (mm) l: Effective length (mm) Also, for the inventive steel and conventional steel, A1 to A10
Fatigue tests were conducted under conditions of 10 to 110 Kgf/mm ² using the above-mentioned coil springs for steel, A15 to A18 steel, and B1 steel, and the above-mentioned torsion bar for A11 to A14 steel. None of them broke even after being repeated 200,000 times. As mentioned above, the steel of the present invention is made by adding appropriate amounts of V and Nb singly or in combination to conventional high-Si spring steel.
Furthermore, in addition to adding appropriate amounts of Cu, Co, and Be alone, appropriate amounts of Cu, Al, or
By adding Co and Ti, or adding appropriate amounts of B and Cr, or adding an appropriate amount of Ta, we have succeeded in further improving the excellent settling resistance of conventional high-Si spring steels. Moreover, the fatigue resistance and toughness required for spring steel are comparable to those of conventional steels, and it has extremely high practicality, especially as a steel for passenger car suspension springs.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the hardness of the inventive steel and the conventional steel after quenching and tempering between 300 and 650°C. Figure 2 is a diagram showing the hardness of the inventive steel and the conventional steel. A diagram showing the results of determining the austenite grain size at the austenitizing temperature by an oxidation method. Figure 3 is a diagram comparing the hardenability of the steel of the present invention and conventional steel. Figures 4 to 8 are the diagrams of the steel of the present invention. and the hardness of conventional steel after quenching and tempering.
It is a diagram showing the amount of settling of a coil spring test piece when HrC is set to 45 to 55.

Claims (1)

[Claims] 1. C0.50-0.80%, Si1.50-2.50 in terms of weight ratio
%, Mn0.50~1.50% and V0.05~0.50% or
Contains V0.05~0.50%, Nb0.05~0.50%, Cu0.20~3.00%, Co0.59~1.00%, Be1.10~
2.00% of the steel for springs, which is characterized by containing one type, with the remainder essentially consisting of Fe, and which has excellent fatigue resistance. 2 C0.50~0.80%, Si1.50~2.50 by weight
%, Mn0.50~1.50% and V0.05~0.50% or
A fatigue-resistant product characterized by containing 0.05 to 0.50% V, 0.05 to 0.50% Nb, and further containing 0.20 to 3.00% Cu, 0.03 to 0.10% Al, and the remainder substantially consisting of Fe. Spring steel with excellent properties. 3 C0.50~0.80%, Si1.50~2.50 by weight
%, Mn0.50~1.50% and V0.05~0.50% or
A fatigue-resistant product characterized by containing V0.05~0.50%, Nb0.05~0.50%, further containing Co00.59~1.00%, Ti0.02~0.10%, and the remainder consisting essentially of Fe. Spring steel with excellent properties. 4 C0.50~0.80%, Si1.50~2.50 by weight
%, Mn0.50~1.50% and V0.05~0.50% or
Contains V0.05~0.50%, Nb0.05~0.50%, Cu0.20~3.00%, B0.0005~0.0100%,
A spring steel with excellent resistance to fatigue, characterized in that it contains one of more than 0.20% to 1.00% Cr, with the remainder substantially consisting of Fe. 5 C0.50~0.80%, Si1.50~2.50 by weight
%, Mn0.50~1.50% and V0.05~0.50% or
Contains V0.05-0.50%, Nb0.05-0.50%, further contains Cu0.20-3.00% and Ta0.05-0.50%,
A spring steel with excellent fatigue resistance characterized by the remainder being substantially made of Fe.